Motor-driven power steering device

Abstract

An electric power steering apparatus can correspond to high output ability, and has a power transmission mechanism of an electric driving section capable of achieving predetermined slip torque in spite of change of operating temperature. Also, a resin gear for power transmission is formed by joining a core metal and a resin with strong bonding force, and has preferable heat-dissipation property. This joining is performed by way of chemical bond according to composite molding technique or adhesive.

Description

The present invention relates to an electrically power assisting steering apparatus.BACKGROUND ARTSIn the operation of an electrically power assisting steering apparatus (hereinafter called as an EPS apparatus) wherein turning force of an electric motor becoming auxiliary steering torque is decelerated by a gear device to transmit it to an output shaft, thereby to assist a driver's power for rotating a steering wheel to steer wheels, e.g., when the steering wheel is rotated up to a stroke end of a rack and the steering is abruptly stopped, the motor can not stop immediately due to inertia force and occasionally, excessive torque is produced in the decelerating gear portion. In order to protect decelerating gears against the excessive torque, conventionally, a torque limiter using a friction plate is provided between a motor and a worm, or a torque limiter using a ring member for applying elastic force is provided between an output shaft and a worm wheel. Thereby, at the time of gene...

AI Technical Summary

Benefits of technology

The present invention relates to an electrically power assisting steering apparatus (EPS apparatus) for vehicles. The technical problem addressed by the invention is to provide a way to prevent excessive torque and wear on the decelerating gears in the EPS apparatus when the driver attempts to rotate the steering wheel quickly, while also ensuring that the gear teeth are not compromised in their strength and accuracy. Additionally, the invention aims to address the issue of excessive torque and wear on the resin gear portion when the diameter of the worm wheel is increased to achieve high output of the EPS apparatus.

Problems solved by technology

As mentioned above, in the conventional EPS apparatus, since the core metal portion of the worm wheel constituting the gear device is formed of solid iron material, there are the following problems.

However, in that case, the weight of the core metal portion is increased and inertial force in steering is increased, so that a driver's feeling in rotating the steering wheel and cutting of the steering wheel are deteriorated.

Also, in compliance with the enlargement of the diameter of the core metal portion, when the diameter of the resin gear portion is enlarged, accuracy of the gears might be worsened to cause increase of noise of the gears in operation since dimensional change of resin due to moisture absorption and temperature is large.

Also, the strength of the gear might be lowered since there is the danger of occurring voids in the resin portion due to injection molding.

Thus, in the conventional EPS apparatus, there are problems in making the diameter of the worm wheel large.

When the ring is worn and the limit torque is lowered because of long-term use of the EPS apparatus, the torque transmission might not be carried out, and thereby the designing is difficult.

However, in the conventional worm wheel of the Japanese Utility Model Publication No. 2556890 and the Japanese Patent Application Laid-Open No. 7-215227, the resin ring X20 is fitted on the irregularities X19a formed on the outer peripheral surface of the metallic core metal 19X, but since the mechanical strength of the resin is inferior to that of metal, the module needs to be enlarged in comparison with a metal gear, and the thickness of the teeth needs to be made larger than that of the metallic portion, whereby there occurs a problem that the decelerating mechanism is enlarged.

Also, the resin with the large thickness is inferior in heat-dissipation ability, and so the thickness of the resin is set appropriately, but there is a problem in that wear of the resin is increased due to heat produced in the engagement of the gears.

Especially, as shown in FIG. 16, when the worm wheel is incorporated in the EPS apparatus and disposed in an engine room, the temperature condition is severe and it is difficult to secure the durability under high temperature environment.

Therefore, there is a problem in that when the resin absorbs water and expands to the extent that the backlash is lost, the operability is deteriorated.

Further, in the worm wheel disclosed in the Japanese Patent Application Laid-Open No. 11-192955, external force generated due to the engagement of the gears acts on the thin resin layer as tensile stress, so that the resin is easily broken and it is difficult to obtain strength.

In this case, the joining of the resin and the core metal relies on mechanical catching, and if only a one portion of the resin is broken, the broken-out surface is enlarged due to the internal stress, so that the binding force is lost.

However, the cost of the MC nylon is very high, and its heat resistance is up to 80.degree. C.

Accordingly, if even a one portion of voids and a weld line is broken, a crack is spread and the fixing force between the core metal and the resin is lost.

Further, although the heat resistance of the resin is up to 120.degree. C., there is a problem in that operability is reduced due to expansion of the resin caused by water absorption.

As structured above, the bonding of the teeth portion of the core metal and the resin becomes strong and even though the thickness of the resin is made small, it is hard to receive tensile stress due to the engagement of the gears.

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